1. Field of the Invention
The present invention relates to a spark plug of a type that is to be mounted on an internal combustion engine, and a producing method thereof.
2. Description of the Related Art
A glass sealable spark plug is conventionally known and it is manufactured by a process comprising the steps of: filling a seal glass material or a combination of seal glass material, a resistive material and seal glass material as a glass sealing material within a space between a center electrode at a top end thereof and a terminal electrode at a rear end thereof which are arranged in a axial hole of an insulator; melting the glass of the glass sealing material by heating; pressing the terminal electrode; and cooling them to solidify.
(A) When glass sealing is effected at a comparatively low temperature using glass sealing materials of low softening point, the energy cost is reduced and yet high operating efficiency is provided.
On the other hand, if the sealed portions (i.e., the top end of the terminal electrode and the rear end portion of the center electrode) are exposed to high temperature, the glass sealing materials will soften and both the terminal and center electrodes will loosen to impair the airtightness of the spark plug.
In addition, the binding force of the electrically conductive substance mixed in the sealing materials and the resistive material drops to produce a higher resistance.
(B) When glass sealing is effected at high temperature exceeding 800.degree. C. using glass sealing materials of high softening point exceeding 750.degree. C., the terminal electrode having a plate of nickel or zinc applied to a low carbon steel is oxidized to corrode in the process of glass sealing.
If the terminal electrode is oxidized to corrode, the plate will come off the steel to cause rust formation on the latter.
If rust forms, the electrical connection of the terminal electrode to the plug cap will deteriorate. In addition, the rust stains the barrel portion of the insulator to cause flashover.